feat(stats): resource metrics dashboard + sites logs/stats

Background collector samples CPU/memory/network/block I/O for every
instance and site on a configurable interval (default 15s, range
5-300s), persists samples to SQLite with a configurable retention
window (default 2h, range 0-24h), and skips ticks gracefully when
the Docker daemon is unreachable. Settings are reloadable without
a restart — each tick re-reads them.

New API endpoints:
- GET /api/system/stats (host snapshot: info + df)
- GET /api/system/stats/history
- GET /api/system/stats/top?by=cpu|memory
- GET /api/projects/{id}/stages/{s}/instances/{iid}/stats/history
- GET /api/sites/{id}/stats[/history]
- GET /api/sites/{id}/logs (SSE + JSON, reuses instance log streamer)

Frontend:
- ECharts added with tree-shaken imports (~180KB gzip) for
  future-proof time-series/gantt/graph visualizations
- CollapsibleSection wraps all dashboard sections (system health,
  daemons, system resources, static sites, projects) with
  localStorage-persisted open state
- SystemResourcesCard shows capacity tiles, workload utilization
  chart with 30m/2h/6h/24h window picker, disk breakdown with
  reclaimable callouts, and top 5 consumers
- ContainerStats and ContainerLogs take a source discriminated union
  so sites reuse the same components as instances; sites detail page
  embeds both for Deno backend debugging
- Settings › Maintenance exposes collection interval + retention
- Docker-unavailable state returns 503 and renders an amber banner
  instead of a generic 500

Full i18n coverage (en + ru) for all new strings.

internal/docker/system.go

@@ -0,0 +1,87 @@
+package docker
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"github.com/moby/moby/client"
+)
+
+// SystemStats is a host-level snapshot combining daemon capacity
+// (NCPU, memory total) with container counts and disk usage broken down
+// by category. Workload CPU/memory utilization is aggregated from
+// per-container samples by the stats collector, not here.
+type SystemStats struct {
+	Timestamp time.Time `json:"timestamp"`
+
+	// Capacity from Docker daemon.
+	NCPU        int   `json:"ncpu"`
+	MemoryTotal int64 `json:"memory_total"`
+
+	// Container/image counts.
+	Containers int `json:"containers"`
+	Running    int `json:"running"`
+	Paused     int `json:"paused"`
+	Stopped    int `json:"stopped"`
+	Images     int `json:"images"`
+
+	// Disk usage by category (bytes).
+	DiskImagesBytes     int64 `json:"disk_images_bytes"`
+	DiskContainersBytes int64 `json:"disk_containers_bytes"`
+	DiskVolumesBytes    int64 `json:"disk_volumes_bytes"`
+	DiskBuildCacheBytes int64 `json:"disk_build_cache_bytes"`
+
+	// Reclaimable disk space by category (bytes).
+	DiskImagesReclaimable     int64 `json:"disk_images_reclaimable"`
+	DiskContainersReclaimable int64 `json:"disk_containers_reclaimable"`
+	DiskVolumesReclaimable    int64 `json:"disk_volumes_reclaimable"`
+	DiskBuildCacheReclaimable int64 `json:"disk_build_cache_reclaimable"`
+
+	// DiskTotalBytes is the sum of the category totals.
+	DiskTotalBytes int64 `json:"disk_total_bytes"`
+}
+
+// GetSystemStats returns a one-shot host-level snapshot. The Info() call
+// and disk usage call are made in sequence. Disk usage failures do not
+// fail the whole call — the result degrades gracefully with zero disk fields.
+func (c *Client) GetSystemStats(ctx context.Context) (SystemStats, error) {
+	info, err := c.Info(ctx)
+	if err != nil {
+		return SystemStats{}, fmt.Errorf("system stats: %w", err)
+	}
+
+	stats := SystemStats{
+		Timestamp:   time.Now().UTC(),
+		NCPU:        info.NCPU,
+		MemoryTotal: info.MemoryTotal,
+		Containers:  info.Containers,
+		Running:     info.Running,
+		Paused:      info.Paused,
+		Stopped:     info.Stopped,
+		Images:      info.Images,
+	}
+
+	du, derr := c.api.DiskUsage(ctx, client.DiskUsageOptions{
+		Containers: true,
+		Images:     true,
+		Volumes:    true,
+		BuildCache: true,
+	})
+	if derr == nil {
+		stats.DiskImagesBytes = du.Images.TotalSize
+		stats.DiskContainersBytes = du.Containers.TotalSize
+		stats.DiskVolumesBytes = du.Volumes.TotalSize
+		stats.DiskBuildCacheBytes = du.BuildCache.TotalSize
+		stats.DiskImagesReclaimable = du.Images.Reclaimable
+		stats.DiskContainersReclaimable = du.Containers.Reclaimable
+		stats.DiskVolumesReclaimable = du.Volumes.Reclaimable
+		stats.DiskBuildCacheReclaimable = du.BuildCache.Reclaimable
+		stats.DiskTotalBytes = stats.DiskImagesBytes +
+			stats.DiskContainersBytes +
+			stats.DiskVolumesBytes +
+			stats.DiskBuildCacheBytes
+	}
+
+	return stats, nil
+}